1. Field of the Invention
The invention relates to a method of scanning a radiation-sensitive surface of a disc-shaped optical record carrier which rotates about a centre of rotation by means of a radiation beam which is directed towards the carrier, the angular velocity of the carrier being substantially inversely proportional to the radial distance between the centre of rotation and the point of incidence of the radiation beam on the carrier. Clock pulses of predetermined frequency and velocity pulses of a frequency proportional to the angular velocity of the carrier are generated and utilized to control such angular velocity.
The invention further relates to apparatus for scanning a radiation-sensitive surface of a rotating optical record carrier by means of a radiation beam, comprising a drive system for rotating the carrier about an axis, an optical system which is radially movable relative to the record carrier, for directing the radiation beam towards the carrier, and a velocity control circuit for controlling the angular velocity of the carrier to a value which is substantially inversely proportional to the distance between the centre of rotation and the point of incidence of the radiation beam on the carrier. The control circuit comprises a clock generator for generating clock pulses and a velocity-pulse generator for generating velocity pulses of a frequency proportional to the angular velocity of the carrier.
2. Description of the Related Art
Such a method and apparatus are used in manufacturing optically readable record carriers which are to be read at a constant linear velocity. During recording on the radiation-sensitive surface of the carrier, for example a photoresist, the scanning radiation beam is modulated in conformity with the information to be recorded. After this, the scanned carrier is subjected to a photo-etching process, yielding a master record carrier having the desired information structure, of which subsequently replicas can be made.
A method and apparatus as described in the opening paragraph are known from U.S. Pat. No. US 4,190,860.
In the method and apparatus described therein the angular velocity of the carrier is controlled by means of phase-locked loop techniques, the angular velocity of the carrier being controlled in such a manner that the velocity pulses remain in phase with reference pulses of a frequency proportional to the desired angular velocity. These reference pulses are derived from the clock pulses by means of a frequency divider having a variable divisor. The divisor is derived from the radial position of an optical system for directing the radiation beam to the carrier. A scanning device provided with such an angular velocity control has the drawback that when the control system is initially rendered operative auxiliary control systems are necessary to bring the angular velocity of the record carrier to a value at which the frequency difference between the reference pulses and the velocity pulses is sufficiently low to establish phase-locking. Moreover, phase-locked loop techniques have the disadvantage that they are not well suited to be carried out by means of a programmable circuit, for example a microcomputer.
A further drawback of the known scanning apparatus is that only a limited number of different scanning speeds can be obtained, because only integral divisors are permitted in deriving the reference pulses. In particular, in the manufacture of optical record carriers, which have to comply with very stringent accuracy requirements, this limitation is a significant drawback.
The last-mentioned drawback can be mitigated by the use of very large divisors for the frequency division. However, this has the disadvantage that the frequency of the clock signal must be very high. These very high frequencies make it virtually impossible to provide angular-velocity control with a satisfactory adjustment accuracy by means of a programmable circuit.